Electric cable.



N0. 873,216. PATBNTBD 1126.16, 1907.

I c. w. DAVIS.

ELECTRIC CABLE.

A'rrmonlox run we. 29. 1906.

(EL- 2.. w. 1%., Q. Qgekd STATES PATENT OFFICE.

OHARLES W. DAVIS, OF EDGEWORTH, PENNSYLVANIA, ASSIGNOB TO STANDARDUNDER- GROUND CABLE COMPANY, OF lffITTSBURG, PENNSYLVANIA, A CORPORATIONOF PENN- SYLVANIA.

. ELECTRIC CABLE.

Patented Dec. 10, 1907.

To all whom it Be it known that 1, CHARLES W. DAVIS,

residing at Edgeworth in the. county of Alleghenyand State ofPennsylvania, a citizen of the United States, have invented or strandedcables' that is, cables with con-.

ductors of relatively large size which to meet 7 the requirements offlexibilityarecomposed of an aggregation of individual. wires, usuallyof copper, laid together without intervemng insulation, so that eachstrand carries a part" of the current, the whole mass of wires beingcovered by an envelop of insulating material, protected or not protectedby an inclosing metallic sheath, as the nature of the insulatingmaterial requires.

The object of my improvement is to in:- crease the current ca g capacityof such stranded cables accor g to the principle and in the mannerhereinafter stated'and described. A

When a current of electricity passes through any conductingmedium acertain portion of the currentis transformed into heat. This heat tendsto raise the temperatureof the conducting medium, the temperaturefinally attained being dependent among other things upon the resistanceof the conductors, the superficial area of the conductors, the amount ofthe current, the temperature of the surrounding medium and the abilityof both conductor and surrounding medium to dissipate by radiation orconduction the heat generated in the conductors.

If the insulating medium around the conductors, and the various mediumswhich in turn surround the insulation are of high thermal conductivity,the energy transformed into heat within the conductor itself, willeasily be dissipated from the conductor with a relatively small increasein temperature in both the conductors and the surrounding medium;

if, on the other hand, the various mediums surroundin the conductors'areof relatively low therm conductivity there will of necessity be arelativel high temperature attained by the cond which lies in closeproximity thereto, the temperature at any point outside of the conuctorsand the material the conductors. I

As the insulating materials commonly made use of, such as rubber, paper,jute, varnished cloth, etc., are deleteri'ousl affected by hightemperature-the actual e ectbeing' dGPGDdG IILHPOD the degree of heat aswell as /4:heleiigthof time it is applied, it becomes vnecessary from apractical standpoint to prevent the insulation from becoming e osed totem eratures of such value as to ring about t deterioration. It willreadily be ductor, made up as I have indicated above, surrounded by theusual insulating wall of some one of the usual insulatin materials, "andplaced in an environment 0 any given character, there would be alimiting value to the current carried, in order that the temperaturefinally attained by the conductor with shall not "be above thetemperature selected as be the maximum to which that kind of insulationshould beexposed.

. It is a well known fact, that as insulated conductors solid asdistinguished from an'nu lar in cross section are increased in crosssectional area, their carrying ca acities, although actually greater,are re atively less. That is to say, under like environment, the lar erthe cable conductor becomes the lower be' its current carrying capacityper unit of cross sectional area for any given limit of maximumtemperature. And this. result is due, among other things, to the factthat the superficial area of the conductor in contact with theinsulating material, or in other words the area through which the heatis transmitted from the conductor to and through the surroundinginsulating walls, is relatively greater in the small sized cable than inthe large sized cable, this being eral area of a cylinder varies as thediameter while the circular cross section varies as the square of thediameter. That is, given two cables, the conductor having in one casetwice the cross-sectional area of the otherthe larger is not ca able ofcarrying a current (other conditions eing the same) twice as lar e-asthesmaller.

t is to increase the carrying capacity of the conductors, for anylimiting maximum temperatures, by taking advantage of my ductors beinglower as it is more remotefromf seen, therefore, that for any given sizeof con and theinsulating material in contact thereclearly explained by aconsideration of the well known geometrical fact that theperiphknowledge of the fact that increased super ficial area oftheconductor increases the radiating .power and consequently reduces In t eaccompanyi wlng's forming a part of this specification I have shown intransverse section three forms or embodiments of my improvement;

My-invention in broad terms consists in' r of anfrom 011'- forming thebody of the cable ro nularfcross section as distinguishe cular crosssectionthat is, in f0 through the center of such a standard conductor aspaceor a hole. Such a construction is indlcated in Figure 1, and,assuming that the crosssectiona area of the'conductor shown in Fig. 1 isthe mile cross sectional area of the solid conductor, I find that themaximum current which may be .passed through the cable of Fig. 1 withoutin'ury to the insulating envelop very materially exseeds in strength themaximum current which the cable having the solid conductor may carry. Itis manifest from the form of the cable that (assuming their crosssectional areas equal) the outer surface of the body of the conductor ofFig. 1 is of-greater extent than the outer surface of the body of asolid conductor of the same current carrying capacity. Accordingly, theheat radiated is in the former case spread over a larger. surface andinto a larger body of the insulating envelop; and for any given currentflow the I temperature attained after a stable condition -is reached, isless than with cable having the solid conductor. I have found that insuch a construction the increase in carrying capacity isat a greaterrate than the increase in cost, within practical limits, and from thisthe practical benefit of the invention becomes apparent. For example, Ihave found in a particular instance that whereas the cost surrounded byan envelop of insulati invention in its broadest terms. 'In Fig. 1 I

show the strands of the cable laid up on a lead pipe, in Fig. 2, acoiled spring forms the hollow core, and in Fig. 3 the core is composedof hemp or jute. Practical conditions mand a cable' suflic ientlyflexible for insertion through surface manholes into undergroundconduits, and the various forms shown possess such flexibilit otherwise,there is no need that the core e flexible.

With a form such as shown in Figs. 1 and 2 a hollow core is present; andin such case a cooling stream of air, water or other fluid may be madeto circulate therethrough, to carry off a portion of the heat generatedin the conductor, and in so doi still greater degree its maximum 0capacity. This additional cooli wilfinsuitable conditions of service efound of ractical value.

t is characteristic of my invention that the strands forming theconductor areso arrangedthat the conductor shall have suchcross-sectionalandperiplieralmto be cap-able of carrying a given currentand of ra 'atingheat at such a rate that the heat generated by suchcurrent will not raise the temperature of the cable to a valuesufficient to injuriously affect the cable.

I claim as my invention:

A stranded cable for electrical purposes com osed of a sheaf or bundleof strands in con uctive contact with one another and material of suchcharacter or nature as to e injured when heated beyond a certaincritical temperature, the said sheaf or the strands composing said sheafbeing disposed in annu lar form of such peripheral extent that the rateof radiation from the conductor when carrying a current of desiredstrength will prevent a rise in. the temperature of the in sulationbeyond the critical point thereof, substantially as' described.

In testimony whereof, I have hereunto set my hand.

CHARLES W.-DAVIS.

Witnesses CHARLES BARNETT, WILLIAM H. WILSON.

to increaseto means

